The invention relates to a system and method to transfer a cryogenic liquid from a station tank system to a recipient tank, wherein at least a part of said cryogenic liquid within said station tank system is stored at a first pressure higher than the pressure in said recipient tank.
Normally bulk liquid CO2 is distributed from various bulk storage tanks, located for example at the place of gas production, —to station tank systems at the customers. The pressure in the bulk distribution chain for liquid CO2, including bulk storage tanks, bulk transport tanks as trailers etc., is normally about 14 to 20 bar. The transport tank takes liquid from the bulk storage tank and delivers it to the station tank system, which means that the pressure in the station tank system will be close to or equal to the pressure in the transport tank.
Applications as for example cooling systems in food transports on trucks often use CO2 as the cooling medium. The CO2 recipient tanks mounted on the trucks, for such cooling systems, normally have an operation pressure of about 8 to 9 bar and with a corresponding equilibrium temperature of about −46° C. With a higher operation pressure in the recipient tank the tank would be heavier and more costly. Further, due to the reduced liquid density and less heat capacity per kg for CO2 at higher temperature and pressure, the cooling capacity per tank volume would be reduced and a larger tank must be used for the same capacity.
Since the recipient tanks are filled with liquid CO2 stored in the large station tank systems, it is then necessary to either reduce the pressure in the station tank or to reduce the pressure of the liquid CO2 when it is transferred from the station tank to the recipient tank. Presently the pressure is reduced before the inlet to the recipient tank by a pressure regulator. In the regulator the liquid CO2 expands and forms a mixture of gaseous and liquid CO2. Both gaseous and liquid CO2 are transferred to the recipient tank. The gaseous CO2 is vented to the atmosphere after passing a vent regulator at the vent outlet system of the recipient tank. This prior art method has the drawbacks that, on the one hand, the filling will take longer since a two-phase-fluid flows into the recipient tank and that, on the other hand, the gas losses are high. It is also not easy to measure the amount of liquid gas, which has been filled into and stays in the recipient tank.
Therefore it is an object of the present invention to provide a method to increase the filling speed and to reduce the gas losses at the transfer of a cryogenic liquid from a station tank to a recipient tank.
This object has been fulfilled by a method to transfer a cryogenic liquid from a station tank system to a recipient tank, wherein at least a part of said cryogenic liquid within said station tank system is stored at a first pressure higher than the pressure in said recipient tank which is characterized in that at least a part of said cryogenic liquid within said station tank system is cooled to a temperature below the equilibrium temperature for said first pressure and that said cooled part of said cryogenic liquid is transferred to said recipient tank.
The station tank system comprises one or more station tanks which are used to store the cryogenic liquid prior to delivering it to a recipient tank.
The expression “cryogenic liquid” shall in particular include liquid carbon dioxide. The main idea of the invention is to provide a system where a part of the stored cryogenic liquid is kept at a temperature near the temperature in the recipient tank. If no pump is used to transfer the liquid gas from the station tank to the recipient tank at least a part of the cryogenic liquid is preferably stored at a higher pressure than the recipient tank pressure. If a pump is used to transfer the liquid gas from the station tank to the recipient tank it is advantageous to store the cryogenic liquid at essentially the same pressure as in the recipient tank. In the later alternative the station tank system might comprise two tanks. The main advantage of the invention is that the gas losses, normally generated as a result of the decrease in temperature, i.e. decrease in pressure, can be reduced or completely eliminated.
Preferably the temperature of said cooled part of said cryogenic liquid differs from the temperature in said recipient tank as little as possible, preferably by no more than 5 K (5° C.).
According to a preferred embodiment the station tank system comprises a first and a second tank. Normally, the pressure in the first tank essentially exceeds the pressure in the recipient tank or the desired pressure in the recipient tank. A part of the cryogenic liquid is transferred from said first tank to the second tank where said cryogenic liquid is cooled down and kept at lower equilibrium pressure.
When the recipient tank shall be filled, the pressure in the second tank is increased by feeding gas from the first tank to the second tank. Then liquid cryogen is pushed by the pressure difference between the second tank and the recipient tank into the recipient tank. The liquid cryogen could also be delivered by a pump from the second tank to the recipient tank. The pressure in the second tank is then preferably equal to or just above the pressure in the recipient tank.
When liquid is transferred from the first tank to the second tank it is advantageous to return gas, resulting from the evaporation of cryogenic liquid in the second tank, back to the station first tank. Since the pressure in the second tank is normally lower than the pressure in the first tank, it is necessary to use a compressor to transfer the gas back to the first tank. The gas leaving the compressor is preferably cooled in a heat exchanger with the same gas before it enters the compressor. Thus the heat transferred to the first tank is minimized.
However, as a consequence of the heat created by the compressor when pumping gas back to the first tank, the pressure in the first tank will increase. In this case it is therefore advantageous to start a cooling machine to cool the gas phase in said first tank and to lower the pressure in the first tank to the desired value.
Preferably the temperature of the liquid gas in said second tank exceeds the temperature in said recipient tank by no more than 5° C., preferably the temperature of the liquid shall be equal to the normal operation temperature in the recipient tank. When it is necessary to refill the second tank with liquid from the first tank it is preferred to use, at the same time, a compressor to pump back gas from the second tank to the first tank. However, the time needed for filling the second tank is then limited by the compressor capacity. If a faster filling is necessary it is also possible to vent some gas from the second tank.
In some cases it might be advantageous to use a cooling machine to cool down and reliquify evaporated gas in the top space of the second tank, instead of using a compressor to return gas to the station tank and hence to lower the pressure in the second tank. However, for cost reasons the compressor solution is normally preferred. An important option to the described two tank solution is to use a pump. instead of a pressure difference to fill the recipient tank. The second tank can be kept at a stable low pressure and low temperature. Gas is only transferred from the first tank to the second tank in order to compensate for depressurization when larger amounts of liquid have been transferred from the second tank into the recipient tank.
An alternative to the two-tank-solution, i.e. the solution of using a second tank for storing a part of the liquid at an extra low temperature, is to create a strong stratification of the liquid in the station tank. In this case only one station tank for storing the cryogenic liquid is necessary. Of course it is also possible to use a station tank system with more than one station tank and to create one or more of these station tanks with the inventive stratification.
Liquid in the lower part of the station tank is subcooled, preferably by indirect heat exchange with a colder fluid, whereas the liquid in the upper parts of the station tank is in equilibrium with the pressure in the head space of the station tank. For example it is possible to subcool liquid CO2 stored in such a station tank by liquid nitrogen.
More preferred is a system where a cooling coil is placed in the lower part of the station tank and the cooling coil is cooled by expanding liquid from the station tank itself. The gas created by expansion and heated by the coil can then be pumped back to the top of the station tank again. The pressure in the station tank, i.e. the gas phase, will be in equilibrium with the surface temperature of the cryogenic liquid, whereas the bottom temperature in the station tank will be as low as can be achieved with help of the stratification. The degree of stratification is dependent on the geometry and insulation of the tank. This results in that the temperature in the station tank decreases from. the top to the bottom of the tank. In case cryogenic liquid shall be delivered to the recipient tank, only subcooled liquid from the bottom of the tank is fed to the recipient tank.
To avoid ice formation in the cooling coil due to the expansion a backpressure regulator might be placed downstream of the coil. Preferably all of said liquid withdrawn from the station tank is gasified during the expansion. To ensure that all liquid has totally changed into the gaseous state a temperature sensor is preferably placed downstream of the cooling coil and upstream of the pressure regulator. The temperature sensor checks that the temperature is above the equilibrium temperature for the pressure set by the pressure regulator.
The gas resulting from the expansion of cryogenic liquid from the station tank is, after it has been used as a heat exchange medium to cool the liquid in the lower part of the station tank, preferably compressed and returned to the station tank to minimize the gas losses. It is even more preferred to compress the gas to a pressure essentially exceeding the pressure in the station tank, cooling the gas and then cooling expanding the compressed cooled and liquefied gas into the station tank. At the expansion of the liquefied gas it converts into a mixture of cooler liquid and gas which cools and/or reliquefies gas in the headspace of the station tank.
The invention is particularly advantageous in the delivery of liquid CO2 from a station tank system to recipient tanks.
The invention will now be illustrated in greater detail with reference to the appended schematic drawings. It is obvious for the man skilled in the art that the invention may be modified in many ways and that the invention is not limited to the specific embodiments described in the following examples.